Hades has a fixed 16:9 aspect ratio. Game developers did implement some limited support for ultrawide monitors as two pillarboxes on each side.

There is a true ultra widescreen patch by nbusseneau. The whole theoretical explanation you can find here[nicolas.busseneau.fr].

• Download patch from github repository releases (depending on you OS select accodingly): https://github.com/nbusseneau/hephaistos/releases
  
• Extract files from archive to the game directory Steam\steamapps\common\Hades
  
• Run Hephaistos executable and follow instructions in popped up console.
  
  Hi! This interactive wizard will help you to set up Hephaistos. Pick an option: 1. Patch Hades using Hephaistos 2. Restore Hades to its pre-Hephaistos state 3. Check current Hades / Hephaistos status 4. Exit Choice:_
  Type 1 -> Enter ⏎ -> Select your screen resolution -> Enter ⏎ -> ??? -> Profit!

If you have some difficulties look through Install[github.com] section on github.

All thanks to Nicolas Busseneau aka nbusseneau.
If you discover any glitches related to ultrawide mod you are welcome to open a new issue[github.com] (with screenshots or videos and your resolution information).